62 



THE HABITAT 



pie Standard. The difference between the two values thus obtained repre- 

 sents the amount of reflected light together with that screened by the epi- 

 dermis. The amount of light transmitted through the leaf may be measured 

 in the same way by using the leaf itself in place of the epidermis alone. 

 The time of exposure is necessarily long, however, and it has been found 

 practicable to obtain leaf prints by exposing the leaf in a printing frame, 

 upon solio paper, at the same time that the epidermis print is made. In a 

 few species both the upper and lower epidermis can be removed and the 

 amount of light absorbed determined directly by exposing the strip covered 



with the chlorenchym. Generally, 

 however, this must be computed by 

 subtracting the sum of the per cents 

 of reflected and transmitted light 

 from ICG per cent, which represents 

 the total light. 



90. Leaf and epidermis prints. In 



diphotic leaves the screening effect 

 of the lower epidermis may be 

 ignored. Isophotic sun leaves, i. e., 

 those nearly upright in position or 

 found above light-colored, reflecting 

 soils, are usually strongly illumi- 

 nated on both sides, and the ab- 

 sorbed light can be obtained only by 

 measuring the screening effect of 

 both epiderms. Shade leaves and 

 submerged leaves often contain 

 chloroplasts in the epidermis, and the 

 above method can not be applied to 

 them. In fact, in habitats where the 

 light is quite diffuse, practically all 

 incident light is absorbed. The rare 

 exceptions are those shade leaves with a distinct bloom. In addition to 

 their use in obtaining the amount of light absorbed, both leaf and epidermis 

 prints are extremely interesting for the direct comparison of light relations 

 in the leaves of species belonging to different habitats. The relative screen- 

 ing value of the upper and lower epidermis, or of the corresponding epiderms 

 of two ecads or two species, is readily ascertained by exposing the two side 

 by side in sunshine, over the slit in the photometer. For leaf prints fresh 

 leaves are desirable, though nearly the same results can be obtained from 



Fig. 15. Leaf print: exposure as before. 

 Sun and shade leaves of Achillea lanulosa, 

 Capitoides aureufn, Antennaria mnbri- 

 nella, Galiuin boreale, and Potentilla 

 propinqua. 



